Radio-frequency identification (RFID) is the wireless non-contact use of radio-frequency electromagnetic fields to transfer data, for the purposes of automatically identifying and tracking tags attached to objects. The tags contain electronically stored information. Some tags are powered by and read at short ranges (a few meters) via magnetic fields (electromagnetic induction). Others use a local power source such as a battery, or else have no battery but collect energy from the interrogating EM field, and then act as a passive transponder to emit microwaves or UHF radio waves (i.e., electromagnetic radiation at high frequencies).
RFID tags contain at least two parts: an integrated circuit for storing and processing information, modulating and demodulating a radio-frequency (RF) signal, collecting (in case of passive tags) DC power from the incident reader signal, and other specialized functions; and an antenna for receiving and transmitting the signal. The tag information is stored in a non-volatile memory. The RFID tag includes either a chip-wired logic or a programmed or programmable data processor for processing the transmission and sensor data, respectively.
RFID systems can be classified by the type of tag and reader. A Passive Reader Active Tag (PRAT) system has a passive reader which only receives radio signals from active tags (battery operated, transmit only). An Active Reader Passive Tag (ARPT) system has an active reader, which transmits interrogator signals and also receives authentication replies from passive tags.
Frequencies at use include:
RangeBandregulations(meters)Data speed120-150 Khz (LF)Unregulated0.1Low13.56 Mhz (HF)ISM band0.1-1  Low to moderate433 Mhz (UHF)Short range 1-100Moderatedevices865-868 Mhz (Europe)ISM band1-12Moderate to high902-928 (North America)
Both electric (E) and magnetic (H) fields at radiofrequencies decrease exponentially with distance from the boundary of a (human/animal) tissue because of energy dissipation after reflection. Penetration depth is defined as the distance in which the power density decreases by a factor of e^−2 (13.5 percent), and is a function of frequency and tissue properties.
At higher frequencies, the radiation fields penetrate a much smaller depth than at lower frequencies. In other words, at the higher end of the RFID spectrum, the radiated electromagnetic field is absorbed to a large extent in human tissue. For example, at 2.45 GHz (microwave oven frequency), penetration depth in tissue is about 2.0 cm, whereas at 10 GHz, it is only about 0.4 cm. At higher frequencies, any heating that occurs is primarily surface heating. Penetration depth for fat and bone is nearly five times greater than for higher-water-content tissues.